If I buy two and a decent price, they're notably cheaper by the time I need the second drive.

If I only buy one drive at a decent price, they're invariable higher than when I bought the first one.

Can I assume you've done the "buy it in the case for $140 and gut it" thing? I've bought several 8TB Seagate drives and put their innards into OWC cases (Guardian Maximus (known now as the Elite Dual I think), for one, as well as a Mercury Elite).